1. Field of the Invention
The present invention concerns a method for monitoring the physiologically effective radio-frequency exposure or in at least one specific volume region of an examination subject in a magnetic resonance measurement (i.e., data acquisition sequence or in a magnetic resonance system, the magnetic resonance system having a radio-frequency antenna structure with a number of individually controllable radio-frequency signal channels for generation of radio-frequency distributions in an examination volume including the examination subject. Moreover, the invention concerns a corresponding radio-frequency monitoring device for implementation of such a method as well as a magnetic resonance system with such a radio-frequency monitoring device.
2. Description of the Prior Art
Magnetic resonance tomography has in recent years become established as an imaging modality in the medical field. The method essentially proceeds in three steps: first a strong, stable, homogeneous magnetic field that produces a stable alignment of the protons in the appertaining body region is generated around the body region. This stable alignment is then altered by radiating electromagnetic radio-frequency energy. Third, this excitation is ended and the magnetic resonance signals arising in the body are detected with suitable acquisition coils in order to make conclusions about the tissue in this body region.
A magnetic resonance tomography system has a number of interacting components, each requiring the use of modern and complicated technologies. As noted above, in a magnetic resonance tomography system the emission of the electromagnetic radio-frequency signals (also called radio-frequency pulses in the following) is important. The radio-frequency pulses output by a radio-frequency power amplifier of the magnetic resonance system are conducted to a transmission radio-frequency antenna structure (also called a transmission antenna or antenna structure in the following) that radiates the radio-frequency pulses.
The antenna structure can be fashioned in many ways. A typical, classical design is known as a birdcage structure with a number of longitudinal rods running parallel to the patient in the longitudinal direction, which longitudinal rods are connected with one another at the end by annular conductors. In order to achieve an optimally good homogeneity of the radio-frequency field in at least one specific volume region, it would be desirable to be able to influence the radiated RF field distribution with optimal precision in a suitable manner at all points of the examination volume. The trend of future developments in the field of magnetic resonance systems therefore is moving toward using antenna structures with a number of individually controllable radio-frequency signal channels, for example with a number of separately controllable inputs or RF feed lines at one transmission antenna and/or with a number of separately controllable antenna elements within the antenna structure (known as transmission arrays). By the passive allocation of these various radio-frequency signal channels, various radio-frequency distributions (for example linearly independent modes) can be generated in the examination volume, the radio-frequency distributions overlapping in a defined manner. Nearly any arbitrary field distribution thus can be achieved by suitable adjustment of the signals on the various radio-frequency signal channels. An example of this is described in DE 101 24 465 A1, which describes an antenna structure for generation of radio-frequency fields in the examination volume of a magnetic resonance system that has a number of separately controllable antenna elements.
Limit values that control the maximum radio-frequency irradiation into a human body have been standardized with the development and establishment of magnetic resonance tomography systems to ensure patient safety. A typical limit value for this purpose is the maximum allowable SAR (specific absorption rate). To adhere to these limit values, measurement values that represent the power of the radio-frequency signals radiated to the antenna structure or from the antenna structure have conventionally been detected in the radio-frequency feed lines in the typical magnetic resonance systems. Power control values are formed on the basis of a number of such radio-frequency power values. These power control values are then compared with a fixed power control value predetermined by a standard, this fixed power control value being selected such that the predetermined SAR value is not exceeded. The emission of radio-frequency signals is then stopped or reduced when a control value exceeds the predetermined threshold.
This means that the maximum allowable SAR is conventionally converted into a maximum allowable power and this power limit value is monitored. However, the physiological effect of radio-frequency energy on a human or animal body depends on, among other things, the frequency and the type of the antenna, i.e. on whether the antenna emits, for example, in a circularly or linearly polarized manner or whether it is, for example, a volume coil or surface coil. Moreover, the effect also depends on the position of the antenna relative to the body of the patient. In the conventional monitoring methods very large safety intervals from the actual critical value therefore must be based in order to ensure 100% safety for the patient. The allowable power limit value consequently generally lies significantly lower than is actually necessary to adhere to the maximum exposure. Since a lower image quality normally accompanies lower radio-frequency power, it would be desirable to reduce this overly large safety interval. It must also be considered that a lower image quality ultimately leads to exposures possibly not offering the desired diagnosis possibilities, or even to exposures having to be re-acquired, which in turn leads to a higher exposure of the patient.
In order to solve this problem, in DE 10 2004 037 840 A1 it is proposed to initially convert the measurement values into exposure values that represent a physiological effect that the radio-frequency signals have on an examination subject exposed to these radio-frequency signals. Exposure control values are then formed based on a number of such physiological exposure values and these exposure control values are then monitored as to whether they reach or exceed an exposure limit value. The emission of radio-frequency signals is limited or interrupted when an exposure control value reaches or exceeds an exposure limit value. In this method as well, however, only the power of the emitted radio-frequency signals is measured.
Given the use of the aforementioned antenna structures with a number of separately controllable radio-frequency signal channels, however, a problem is that the radio-frequency power that ultimately acts on the examination subject (i.e. the patient or test subject) in most cases does not agree with the sum of the individual powers at the various radio-frequency signal channels, since constructive and destructive overlapping of the electrical fields of the individual radio-frequency signal channels occurs. As before, such systems must therefore be operated with overly large safety intervals.